Can quantum measurements change the past?

24:49Exactly. And that's a much more powerful way to solve physics problems. It's the foundation of all of modern physics, but does not imply retrocausality. So the important thing to take away there is that you can have physics which depends on future conditions which are not controllable, and so you won't get paradoxes. Got it. All right? So that's the crucial thing. And so what does that mean about the universe, right? Like, is the universe a computer or does the universe like use minimal principles to figure out after the fact what happened? Right. And so the answer is we don't really know. And things get really sticky once you move to the quantum version of Newton's computer.

25:26OK, yeah. So we haven't broken my brain yet, which must mean we're not done. So let's go ahead and take a break. And when we get back, we'll work on breaking the brain.

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28:21All right, we are back and we are moving to the quantum version of our explanation. So put on your quantum hats and gear up. Yeah, so now let's think about whether this idea of Newton's computer really works to describe the universe once we start to have the rules of quantum mechanics. Because we know that Newton's classical theory of physics, where you have all the information about the universe and you can predict it perfectly into the future if you have the laws of physics, doesn't really work because we don't have the information. So what happens when we try to make a quantum version of Newton's computer?

28:57And here I have to give a shout out to Ken Wharton, a listener who's also a professor in quantum foundations at San Jose State, who sent me an email after the Entanglements episode and a really fun paper to read, which outlines a lot of these fascinating ideas. Thanks very much, Ken, for sharing your insights. Oh, Ken was my husband's Zach's physics teacher when he was at San Jose State. Zach got like four ninths of a physics degree before his comic took off and he didn't have enough time to go to school anymore. But Zach said Ken was a fantastic physics teacher. Yeah, he's super fun. I had a great chat with him about retro causality, which inspired a lot of the ideas in this episode.

29:35So thanks very much, Ken. Awesome. Side note. Hi, Ken. Zach really did say that you were a great physics teacher. We talked about it over coffee a few days ago after you had the chat with Daniel over Blue Sky. All right. So there's some issues here about thinking about the universe as a computer if it really is quantum mechanical. Number one, we don't really know what was the initial state of the universe. We don't know the current state of the universe. We can't measure it precisely. We can't tell the location and direction of every particle because of the uncertainty principle. And no matter how far back in time, that's true.

30:07And the initial state of the universe can be undetermined in that same way. So there's like a fundamental fuzziness which prevents the universe from operating as a computer in that way. And the second is that its predictions are probabilistic, right? It does not tell you what's going to happen. It tells you the probabilities of various things happening. So in that sense, the past doesn't determine the future completely. And so in that way, it gets kind of fuzzy if you want to hang on to Newton's idea that the past predicts the future, because we don't know the past perfectly, and it doesn't actually predict the future in quantum mechanics.

30:41Did I follow that? The initial state's not really known. Okay, I did follow that. Why should I care? Yeah, you might think, well, it's academic, doesn't really matter. It predicts it in a different way, right? We're predicting probabilities instead of actual outcomes. Isn't that fine? Well, it matters in situations like the one we talked about a few episodes ago for entangled particles. And this is like what really troubled Einstein. So the scenario there is you have like two particles that have been created by some process. And so they're entangled. Like they can't just have any random state.

31:13Maybe they have to conserve angular momentum. So one has to be spin up and one has to be spin down. Like if you have a spin zero state like a photon, which turns into an electron and a positron, and those are spin one half, then one of them has to be like spin minus one half, one of them has to be spin plus one half. And so that means that either your electron is spin up and your positron is spin down, or your electron is spin down and your positron is spin up. They can't be both spin up and they can't be both spin down. So in that sense, they're entangled. The conditions you've used to create these two particles means that there's a connection between the states of the particles.

31:46With me so far? Yep. Okay. And that doesn't seem too weird. It's like if I have a bag and I know there's a red ball and a blue ball and Kelly and I each draw a ball out of the bag, then by looking at my ball, I can tell, oh, I have a blue ball, Kelly must have a red one. Or if I have a red ball, Kelly must have a blue one. That's not so weird. The weird thing is quantum mechanics says that it's not just that I don't know which spin my particle is, that it's not determined, right? that my particle is in this uncollapsed state of a mixture of spin up and spin down, and so is Kelly's. But that weirdly, when I measure my electron and I say, oh, mine's spin up, that that changes Kelly's electron to now be spin down because mine is spin up.

32:32So when I make my measurement, it goes from undetermined to spin up. Kelly's goes from undetermined to spin down. And the weird thing is this is true even if our electrons are really far apart, like we can move them light years apart. And the rule here tells us that this happens instantaneously across space and time, right? As soon as I make my measurement, Kelly's changes from undetermined to spin down. I remember this from our entanglements episode. Exactly. And you might ask, well, how do you know? How can you prove it? How do you know that it wasn't really just spin up and spin down the whole time?

33:05And the entanglement episode goes through Bell's experiment, which proved this. It proved that there is no local hidden information. that there's not like some feature of mine which meant it had to be spin up the whole time and some feature of Kelly's which meant it had to be spin down the whole time, that it really is undetermined and the universe chooses at the moment one of us makes a measurement. So this is Bell's experiment and go back and dig into that if you want more details about it. And a couple crucial things to understand about Bell's experiment is number one, it only proves that there's no local hidden information.

33:38So you can have some global pattern. There are theories like Bohmian mechanics or whatever that say that it's controlled over space and time. There's that global information. Those are more fringe theories. People believe the universe is local. So there's that as a caveat to Bell's experiment. Another important caveat to Bell's experiment, and this is what Ken Wharton's paper is all about, is that Bell also assumes that there's no dependence on the future, that the universe operates like a quantum computer, that nothing that happens in the future can determine anything that's happening in the present or in the past.

34:11And you might think, yeah, I'm cool with that. No big deal. Of course, things can't go into the past because that would create paradoxes and we don't want that. So we put a ball goes in each of our bags. That's the past. At some point, one of us opens the bag. That's the present. And we would be talking about at some point in the future, something happens that impacts both of those moments that I just talked about. That's what we're talking about in the future, right? Yeah. Or to spoil it a little bit, Bell is saying, it's not that when I make my measurement of my ball and I just start reminding spin up that it goes back and changes the past to mine was always spin up.

34:50Goes back and changes the past. So yours was always spin up. Okay. Yeah. This is what physics does to me. I'm like, let's go ahead and make sure I understand the definitions of past, present, and future. Yeah. And so you might be like, all right, that's cool. Of course, the future doesn't affect the past. And you want to hang on to that. But even if you do, there's already a problem with this experiment, which is this instantaneous component, right? Like in the version of the story I just told you, that seems weird, right? To have instantaneous collapse over time. I make my measurement and Kelly's changes instantaneously.

35:23And that's a problem because in relativity, this concept of simultaneity, like things happening at the same time is not universal. It's frame dependent, right? Like if you have two events, A and then B, I might think that happened at the same time, but Kelly moving at a train at half the speed of light going past me can see A happening before B. And Zach moving a train the other direction can see B happening before A. And so relativity can scramble the orders of things. And so already, if you're going to insist on instantaneous collapse across time, then you're already allowing for a form of retro causality.

36:02Because if I make my measurement and then Kelly's is instantly collapsed, somebody moving a train past us could see Kelly's being collapsed before I make my measurement. No more trains. Everyone has to stand still. This is too confusing. Yeah, exactly. And so the point here is that even if you don't want to allow retrocausality, this concept of instant collapse when combined with special relativity says that you're already in dangerous territory. You're already potentially having information moving backwards in time. Now, we're saved here because all this information is hidden. Like, remember, in the case of the classical example of the minimal principles, we're not saying that it's changing the past by solving this over time, knowing the initial state and the final state and solving for the intermediate.

36:54We're not saying that that changes the past because you can't control the final state. There's no way to use that to change what's happening. Here, there's no signaling back in time because the thing that might be retrocausal, the thing where the future might affect the past is hidden. It's the same reason why we can't have instantaneous communication. Like when I make my measurement of my electron, it goes from undetermined to determined and yours far away in space, light years away, maybe goes from undetermined to determined, but we can't use that to send information faster than light, even if that collapses instantaneous.

37:28And the reason for that is that you can't tell that I've made my measurement, I make my measurement. It goes from undetermined to determined. You're looking at your ball. You don't know if I've made my measurement, and there's no way for you to tell. All you can do is measure your ball and get like, oh, I got spinned down. I wonder if that's because Daniel collapsed it before I measured it, or if my measurement collapsed. You cannot tell. So the fact that something has happened instantaneously across space and time is hidden from you. That's the reason why we can't use it for communicating instantaneously across space and time.

37:59That's the thing science fiction always gets wrong. And that's crucial, right? That's what prevents the paradoxes and the signaling backwards in time. That's why this isn't a fatal flaw, why special relativity saying from some perspectives, what's happening here is retrocausal, is things in the future affecting things in the past. From some observer's point of view, Kelly's status collapsed before Daniel makes his measurement. That's not a problem because no information is going backwards in time. It can't be used to create any paradoxes because that information is hidden from Kelly. Even if Kelly's present depends on Daniel's future, it's not in a way that she can tell.

38:37And so no information can go backwards in time. So the punchline is you can actually have retrocausality in quantum mechanics without ruining the universe. Okay, all right. Okay, so I think I get this. So, okay, so does that mean that the only way you could get retrocausality would be like, Like, you know, for example, we're opening our bags and we're passing each other really quickly on like a train. And they're like momentarily this retro causality thing happens, but we don't realize it, but it happened. Yeah. And that's it. Exactly. Okay. And so you could take a whole different view of this if you allow retro causality.

39:13Because remember, Bell's experiment, which in principle proves that the collapse is instantaneous and goes from undetermined to determined, relies on an assumption. The assumption is no dependence on the future. So Ken Wharton's amazing and hilarious and beautiful paper says, you know, he hasn't actually ruled out retrocausal theories. Let's tell the story another way. Because he just assumed no retrocausality, right? So what happens if you allow retrocausality? Well, you know what? I'm dying to know, but I'm going to make everybody wait. So let's find out what happens if you add retro causality after the break.

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42:35All right, we're back. We're going to talk about what happens when you add retro causality to Bell's experiment. But first, let's remind ourselves real quick, what did Bell do in his experiment? So Bell has this experiment where you have these two entangled particles and they're separated across space and you measure one of them and collapse it. And you ask whether the other one is collapsing right then or whether it was already collapsed and you just didn't know it. He's trying to distinguish between the cases of, I don't know, but it's already determined, and I don't know because it's not yet determined.

43:07And he did this amazing series of experiments where you have these particles and you make different measurements on them, and there's different random elements of those experiments to show the quantum correlations prove that things are undetermined. It's a really subtle experiment, and there's no individual experiment that's like, this one result is a smoking gun of quantum mechanics. It's more like a pattern over many iterations of the experiment proves about how the information is stored in the system. And the takeaway from Bell's experiment is that there are no local hidden variables. But again, Bell assumes no dependence on the future, right?

43:42So the typical conclusion we draw from Bell's experiment is what we often say about quantum mechanics, that things have two possibilities and they can maintain those things. And when I measure my particle, it's gone from having those two possibilities to deciding whether it's up or down and that yours collapses instantaneously across space and time, right? That's the typical conclusion. But there's another way to view these by allowing a little bit of retro causality because Bell didn't rule that out, right? Bell assumed no future dependence. But what if we tell a different version of the story and it goes like this?

44:14I make my measurement and my particle goes from undetermined to determined. and my particle goes from undetermined to up. And that propagates backwards in time so that my particle was always spin up and your particle was always spin down. Okay. And so that part is going backwards in time, right? And it yields the same results. Mine is spin up and yours is spin down. You don't have this moment where your particle collapses instantaneously across space and time. Instead, we allow the information to propagate backwards. And Bell's experiment, again, does not rule that out because he has just assumed that this can't happen.

44:53And so when you assume it, you close yourself off to that conclusion. While it's understandably weird to think about particle states depending on the future, it actually solves some of the conceptual problems that we laid out before. In the instantaneous collapse model, we had this bizarre frame dependence when the order of events depends on the observer and their like velocity relative to the experiment. Plus we had this instantaneous collapse across space-time without any sort of physical mediator capable of doing that. So the retrocausal model is frame invariant because it has this local hidden variable that is future dependent.

45:30So my particle was always up and Kelly's particle was always down because they both depend on my future measurement. So no matter what your velocity is relative to the experiment, you see the same thing. That's pretty cool. And there's no need for unexplained transit of information instantaneously. And it also does not create paradoxes. It's in principle information flowing backwards in time, but it's hidden information. For the same reason that you can't use entangled particles at a distance to send information back and forth in the other view of this experiment, you can't use it here to send information backwards in time because the information that's going backwards is hidden.

46:12Like you still can't tell whether I've made my measurement, whether it's collapsed now, whether it's collapsed backwards in time. You just have a particle, you make a measurement, you get spin up or you get spin down. And I should say that propagated here is really just a figure of speech. It's not physical. There's no sense in which my particle was undetermined earlier and then information propagated back and later my particle was spin up. My particle was always spin up. It's just that the hidden information there depends on a future event. You can't tell that that information propagated backwards in time.

46:46So there's no way to use it to create a paradox. This might be a silly question, but so the particles that they were looking at, they knew they weren't, I don't know, for example, traveling quickly on trains past each other, right? So were they able to control for the possibility of relativistic effects because of how they were measuring the particles and thus control for the possibility of retrocausality? Or am I missing something? You can't control for that because you can always have an observer moving past the experiment. Like even if they keep the particles at rest relative to the earth and all their experiments are making these measurements on the earth, so there's no relative velocity, you could always put Zach in a rocket ship flying past the experiment with no change to the experiment and he would see it operating in a different order because of his velocity relative to the experiment.

47:33But can't I be like, during this experiment, sit your butt down and do not move because we're doing science. Yeah, but it doesn't matter if nobody does it. It just matters whether it's possible. And it's also always some particle out there moving past to whatever speed. Okay, got it. But I like your sort of school teacher approach. Like, hey, everybody, don't break the universe now, please, I'm trusting you. That's right. You can't trust Zach to just sit still, you know? Yeah. So let's again delineate the two scenarios here. The canonical version without retrocausality says that I measure my particle and then instantaneously across space and time, yours goes from undetermined to determined.

48:14And we already pointed out that sort of implies retrocausality because of the frame dependence of special relativity and the dependence on the order of events. The retrocausal version says, when I make my measurement of my particle, that information propagates backwards in time and makes it so that our particles were always the way we ended up measuring them. So it determines that hidden information. So it is local hidden variables, right? And you might think, hold on a second, Bell ruled out local hidden variables. Those are not allowed. But Bell assumed no dependence on the future. and because this is hidden information, it does not create paradoxes.

48:53So I think it's super fascinating because it opens your mind up to like a really weird way for the universe to potentially operate that's actually still consistent with experiments. And it's just like a different interpretation of the same data. That is wild. Yeah. So now I want to take this idea and pick apart a famous experiment that's often used to claim that quantum mechanics does to send information into the past in a way that could be used to signal, which is very, very wrong. And it's a fascinating experiment. It teaches us a lot about quantum mechanics, but it's often misexplained in this same context.

49:28So I want to take it apart. All right. What's it called? Because every great experiment has a name. It is very cool. It's called the quantum eraser. And it's one of these great experiments that tries to understand how information collapses and what an observer is. And the idea is to, instead of making your measurement directly, like Daniel is a big classical object, he's going to use some detector which collapses the wave function. What if we could try to extract this information without collapsing these uncertainties? What if we could try to get the information and maintain the quantum uncertainties?

50:04Because in quantum mechanics, you have this distinction between classical objects which collapse things from superpositions into a single choice, and quantum interactions which don't collapse things. Like if I have my electron and it's spin up, I can send another electron to interact with it, and that whole system can remain in a superposition of possible states. It doesn't have to collapse. Not every interaction leads to a collapse of superposition into a single choice. Only when you interact with a classical object, things which cannot be in a superposition, like me or a detector or my eyeball, big classical things.

50:40So the idea is like, let's do the same experiment where you have two particles and they're entangled. So one of them went left and one of them went right, or one of them is spin up, one of them is to spin down. And instead of interacting them with some big detector, which forces them to choose, instead entangle them with another particle, another quantum object. So you have no collapse, right? And so this is typically done in like the double slit experiment. In the double slit experiment, you have like electron that's shot at two slits. You don't know if it goes through the left slit or the right slit.

51:11And so that creates an interference pattern because the probability for it to go to the left interferes with the probability for it to go to the right. And you get this interference pattern. But if you put a detector on it to measure which way it went through, the interference pattern disappears. That's the classic double slit experiment. So in this case, we replace the detector with a quantum detector, some electron or something which interacts with a particle in the experiment, but not in a way that collapses its wave function, right? It stores the information about whether the particle went through the left slit or the right slit, but in a quantum object, not in a classical detector.

51:46Is this a thought experiment or an actual thing? This is an actual experiment you can do. Oh, okay. So now you, in principle, the information is encoded in this other electron, right? And it hasn't collapsed yet. So you've preserved the uncertainty while encoding the information so you can find it later. So the way this story is typically mistold is the following. And again, this is incorrect. It says that the particle continues through the double slits and you get the interference and that you can go later to measure the information, the quantum information you've stored in this electron. You can then go make a measurement to figure out which slit it went through after the particle has already hit the screen, right?

52:25So you're like, oh, it's too late for it to change. Now I'm going to go figure out which one it went through. And the incorrect version, which you see all over popular science, is that this goes back and changes what happens on the screen. That like you have an interference pattern on the screen, and then you go and you measure it, and the interference pattern disappears. To be clear, this would be retro signaling if it actually happened, but it doesn't. So the incorrect version of the story is that you take an action in the future, well after the particles have already hit the screen and made an interference pattern, maybe a year later, you decide you want to know whether they went left or right.

53:04So then you access the stored information in that quantum detector. And the incorrect story is that the collapse then happens backwards in time, going back to change the pattern on the screen from a year ago from interference to no interference, which is like, what? That's crazy and spooky. That's because that's not what happens. That's literally just not the outcome of that experiment. It would be really fascinating if it were, but it's not. What happens when you do this experiment is when you use the electron to figure out which slit it went through, the interference pattern disappears already.

53:39To be very clear, once you add the quantum detector, the interference pattern disappears. Not when you open that detector a year later, but as soon as you add it to the experiment, whether or not you look at the information. If you have the quantum detector, you never see the interference pattern, whether or not you access the stored information in the quantum detector, because you've measured that information. You've encoded it in an electron instead of some big classical detector, but the interference pattern does disappear on the screen. You do not see an interference pattern because that information has already been extracted from the experiment.

54:16Wait, so there was an interference pattern, and then you look at the electrons, and it goes away. There is an interference pattern on the screen. Then you add the electron detector and the interference pattern goes away. You don't have to look at the electron for the interference pattern to go away. Oh, just having the electron detector makes it go away, even if you don't look at the response in the electron detector? Exactly. So having a classical object add an electron detector is all it takes, even if the electron detector is a quantum detector. That's right. Even if it's a quantum detector, it still collapses the interference pattern.

54:55What's cool is that you can then erase that information by scrambling it somehow, or you can go back and you can sort of recover the interference pattern after the fact. You can use the information in those electrons to pull out an interference pattern after the fact, but it's not like an interference pattern appears on the screen. You can use the information in your quantum detector to separate the particles on the screen into two groups. And if you pull those apart, each of those will have an interference pattern in them. So there's no retro causality here. It's a fascinating experiment because it shows how quantum information propagates through the experiment.

55:36But the way it's typically told, that by using a quantum detector instead of a classical detector, you are sending information back in time. Because if you look at the outcome of your quantum detector later, the interference pattern disappears. That's not true. The interference pattern disappears as soon as you inject the detector into the system, quantum or classical. Come on, man. This stuff is crazy. Yeah. Yeah, exactly. And so this stuff is weird, but there's a retrocausal interpretation of this. The idea is that you make a choice whether to look at this quantum detector or not, whether to collapse the quantum detector's information, and that sends information back in time to determine what happened in this experiment.

56:19But again, it's hidden information. It's not information you can use to send messages or to do anything else. And there's another famous example we don't have time to go through, which is called the delayed choice version of the double slit experiment, which is when you move the detector really close to the screen. And so basically you're deciding whether to detect the particle just before it hit the screen, well, after it had to decide which slit to go through or whether to make an interference pattern or not. It's a really cool experiment. But in both cases, you can interpret these experiments in a retrocausal way by saying that information propagates backwards in time, but again, only to control a local hidden variable.

56:58So basically, retrocausality allows you to reintroduce local hidden variables in our experiments by allowing information to propagate backwards in time, which is kind of uncomfortable and kind of weird because it violates our sense that the universe should flow from the past into the future, and that we like to think about the universe as a computer, that it's calculating things on the fly, but we don't know how the universe works. And it's certainly not restricted to doing something that makes sense to us. No, it's not. No, it's not. And another question you might ask is, well, could we tell the difference between the one scenario where things really are undetermined until you measure them and then they collapse instantaneously across space and the retrocausal version where things propagate back in time?

57:48And I asked this question to Ken Wharton, Zach's physics professor, who's a philosopher of quantum foundations, and he said, maybe there will never be an experiment. And, you know, one issue is that we don't have like a full theory of retrocausality. Ken's paper, for example, just points out that this is allowed in theory. So he doesn't have a full theory that incorporates all of this and predicts everything. And the problem is that all the experiments that we could set up to test this idea would also allow for signaling backwards in time, which would create paradoxes. So those things definitely don't work.

58:21Wait, I thought they wouldn't allow for signaling back in time. Yeah, exactly. So the experiments we set up to try to test this would also test for retro signaling, which we already know can't work. So we don't know how to test for this just retro causality without retro signaling. Got it. Okay. So yeah, we don't know how to test for retro causality. But the good news is that if we take a retro causal formulation of quantum mechanics, it might make it easier to solve the big open problem of quantum gravity. One of the big sticking points there is that general relativity demands to know where things are at all times.

58:55Where are the masses in space? So we can decide how much it's curving. And traditional quantum mechanics says you can't know that. Some things are undetermined. But retrocausal quantum mechanics says that there are local hidden variables that do determine where the particles are. So if those can be made like covariant, then you can marry that with ordinary general relativity space-time and maybe make some progress on quantum gravity. And so the headline is that a lot of the popular science quantum mechanical articles about how quantum mechanics changes in the past are wrong because they imply that you could send information back in the past, which could create paradoxes.

59:34The more interesting but nuanced bit is that some interpretations of quantum mechanics are consistent with a form of retrocausality, which changes local hidden variables in the past in a way that does not allow for information to be propagated into the past to create paradoxes, but is a fascinating insight into how the universe works, whether it really is a computer, whether it follows these minimal principles, whether you should think of the universe as like a block. Maybe the universe is not figuring it out as it goes. It's just like one big physics problem and some physics major at the end of the universe is going to solve for the whole thing, given the initial and the final state.

1:00:11Oh, God, I hope he doesn't mix up the pluses and the minuses. Physics major is not famously good at that. Yeah, exactly. Good luck. Yeah. All right, so thank you for taking this journey with us forwards and backwards in understanding. It turns out the universe is far weirder than we imagined and maybe far weirder than we could ever understand. Well, that was trippy and fun. Thanks, Daniel. Until next time, Extraordinaries, have a good one.

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